"Every book, every volume you see here, has a soul. The soul of the person who wrote it and of those who read it and lived and dreamed with it. Every time a book changes hands, every time someone runs his eyes down its pages, its spirit grows and strengthens
Short answer: you can't, in the sense of a "delete book" function. But I'm wondering why you would want to, as the reasons may lead to possible workarounds.
If you no longer have the books in question, you can make release notes to indicate where they went - or, if you don't remember, just make a journal entry to say "No idea where this one is". If you set the status to "traveling" (this is automatic with release notes, manual with journal entries), it will be easy for you to tell which books on your shelf are no longer in your possession.
If some of the books on your shelf were never labeled, then you could re-use those BCIDs for some other book - use the "edit book details" link to change the title/author/category information, and edit the journal entry to change your comments. But you should not do this if the books you originally registered may still be out there somewhere with BC information in them - you never know when somebody will find and journal them!